The present invention relates to an electronic camera which can take both still and motion pictures. More particularly, the invention relates to a stop control apparatus, hereinafter referred to for simplicity as a stop, for use in an electronic camera.
In an electronic camera, an image pickup device such as a solid-state image pickup element is used to obtain an electrical signal varying in magnitude according to the brightness of an object being imaged. The electrical signal thus obtained is stored on a magnetic disc or in a semiconductor memory. The electronic camera is advantageous in that it requires no photographing film which must be developed chemically, and the picture can be reproduced quickly with a television set. A variety of such electronic cameras have been proposed in the art to replace conventional cameras using film.
As mentioned above, an electronic camera uses an image pickup device instead of a film. However, the other essential components of the electronic camera, such as an exposure control device, can be the same as those of a conventional still-type film camera. That is, in an electronic camera, the exposure can be controlled utilizing a photometer, shutter, and stop of the same general construction as in a still camera.
An electronic camera, as described before, uses a lens and image pickup device. Therefore, if the electronic camera is coupled to a motion picture recording device such as a video tape recorder instead of a still picture recording device, it is theoretically possible to use it as a video camera for taking motion pictures. However, in performing video photographing operations with such an electronic camera, the degree of opening of the aperture must be continuously adjusted in accordance with the brightness of the object at all times because the exposure time of the image pickup device is fixed to a value corresponding to one frame period or one field period. Accordingly, in a conventional video camera, a diaphragm opening and closing mechanism connected to an electromagnetic drive device such as a galvanometer is provided, and a luminance signal corresponding to the average brightness of the object is applied to the electromagnetic drive device thereby to control the diaphragm. On the other hand, in a conventional still camera, the stop is implemented with a mechanical arrangement using an aperture ring. Such a mechanism cannot continuously control the aperture in response to an electrical signal, and hence such a mechanism cannot be applied to an electronic camera capable of both motion and still picture photographing.
Furthermore, the above-described electromagnetic diaphragm control device employed in a conventional video camera cannot maintain a preset aperture value, as can be done by a conventional aperture-priority still camera, and cannot automatically set an aperture value in accordance with a selected shutter speed, as can be done by a shutter-priority still camera. Therefore, if such a device were used as a stop for a still camera, it could not perform all the aperture control functions which conventional aperture priority and shutter priority cameras perform.
In other words, it is extremely difficult for an electromagnetic diaphragm control device to set aperture blades to a desired position instantly when the shutter is operated, and neither the stop control apparatus of a video camera nor that of a still camera can satisfactorily function as a stop control apparatus in an electronic camera which is capable of both motion and still picture photographing.